The widespread adoption of composite materials in aircraft structures has led to significant performance improvements. The composite material incorporates two or more component materials, each of which retains its individual identity, as phases into a single composite material. The resulting composite material is engineered to realize the most-advantageous properties of the various component materials, while minimizing or avoiding their disadvantageous properties.
In a typical case, strong fibers are embedded in a polymeric matrix. An example of such a composite material includes carbon or graphite fibers embedded in a polymer matrix such as an epoxy. The high strength and modulus of the fibers are incorporated into the composite material. These materials exhibit much higher stiffness-to-weight and strength-to-weight ratios than do conventional metallic materials.
However, many of the composite materials and other low-ductility materials exhibit a low resistance to mechanical impact damage. If, for example, a tool is dropped on a panel of the composite material or a stone is kicked up and impacts the composite panel during takeoff or landing, the resulting damage may serve as a failure-initiation site. That is, local damage at the initial impact site may propagate and lead to premature failure of the composite material. The occurrence of mechanical impact strikes may be difficult to identify in many cases, because they are not visible to the unaided eye even though damage may have occurred below the surface of the composite material. Mechanical impact damage is less of a concern for conventional metallic materials that typically have higher ductilities than do the composite materials, although it is a problem. Such metallic materials inelastically deform and readily indicate mechanical impacts, as for example by dents in the surface.
In recognition of the potential for mechanical impact damage of the composite materials, testing and design standards are imposed on composite material structures beyond those required for metallic structures. One part of these standards is a damage-tolerance factor, sometimes termed a “knockdown factor” or a “barely visible impact damage (BVID) factor” in the industry, by which performance values of the composite materials are reduced for the purpose of establishing allowable design standards. The damage-tolerance factor reduces the allowable mechanical property values (i.e., stiffness, strength) to which the composite material may be designed in light of the potential for undetectable mechanical impact and other types of difficult-to-detect damage. The use of the damage-tolerance factor thus reduces the weight advantage that the composite materials otherwise enjoy. As a result of the application of the damage-tolerance factor, the composite material components must be made heavier than would otherwise be the case, so that the components are certain to retain their required strength even after mechanical impact damage has occurred.
There is a need for an improved approach to the problem of mechanical impact damage in composite materials and other low-ductility materials, including both indicating the presence of such damage and also in the setting of design standards. The present invention fulfills this need, and further provides related advantages.